Alternaria Leaf Blight in Carrot

Carrot leaf blight is a disease commonly found in carrot crops in Western Australia. It is usually caused by the fungus Alternaria dauci and occasionally by A. radicina. Another fungus, Cercospora carotae, causes leaf spotting of carrots. Both Alternaria and Cercospora can weaken leaves and in severe cases can defoliate crops.

Mechanical harvesting is difficult when leaves are weakened by blight. Alternaria dauci is more common in autumn and winter, and Cercospora carotae is more common in summer. It is possible for both types of fungi to be present at the same time in a crop.

Symptoms

Alternaria dauci appears on leaves as small variously-sized dark brown to black lesions. The lesions often appear on the edges or margins of the carrot leaf. In severe cases, the lesions expand, causing the leaflets to turn brown, shrivel and die. The leaf may have a scorched appearance.

Alternaria dauci on carrot leaf

The petiole or leaf stems can also become infected and develop brown irregular-shaped lesions.

Generally, the older, lowest leaves of a carrot are affected before the upper younger leaves. The disease will first be obvious in carrot crops as irregular patches or ‘hotspots’ of diseased leaves.

Cercospora carotae appears as small, almost circular, brown spots that are often surrounded by a yellow border. Generally the upper, younger leaves are affected first. C. carotae is not as prevalent as Alternaria dauci.

In the past, outbreaks of Cercospora had symptoms that were indistinguishable from those caused by Alternaria.

Alternaria dauci symptoms in close up

Although symptoms of bacterial blight (Xanthomonas campetris pv. carotae) can be confused easily with those of alternaria leaf blight, the lesions of bacterial blight are smaller, with a characteristic yellow border. However, bacterial blight has only occasionally been observed in Western Australia and not in recent years.

Root scab complex or carrot scab may be caused by seed-borne Alternaria or severe blight outbreaks in the field. This disorder is characterised by thin corky black lesions arising on the secondary root nodes on the carrot. Fusarium species can usually be isolated from scab lesions on carrots taken from the field, but evidence suggests that the Fusarium may be a secondary invader.

Spread

Leaf blight spores are spread by water, wind and machinery. The spores may come from other diseased fields or from debris of decomposing carrot leaves.

Alternaria dauci can be introduced on infested carrot seed. Spores produced on infected plants are spread rapidly during wet windy weather.